TL;DR
Picture 0
Picture 1
Certian cheap 18650 cell-holders (Pict. 0, Pict. 1) have a critical design flaw that can cause them to short out your cells.
If you have used these in projects,replace them!
Picture 0
Picture 1
Certian cheap 18650 cell-holders (Pict. 0, Pict. 1) have a critical design flaw that can cause them to short out your cells.
If you have used these in projects,replace them!
Picture 2
A simplified crosssection of the top of an 18650 cell. Metal parts in gray, cell wrapper in magenta, insulating material in blue, cell contents in orange.
Picture 3
The positive terminal of the holder, pictured when looking straight onto the positive end. Orange represents the metal connector piece, blue where the negative rim of an 18650 is located, red the positive cable.
The top of an 18650 cell is thought of as the "positive" end, but actually, it also has a negative rim. That rim is usually covered by a plastic foil "wrapper" which covers the entire cell. (See Pict. 2)
The foil makes sure that a conection can only be made at the ends of the cell, though usually there isn't any problem using a cell without it.
I say usually, because the problematic cell holders have a tiny metal piece (Pict. 1) at the positive end, which is part of the holders positive pad, and crimps onto its red cable ... right where the negative rim on an average 18650 is located! (See Pict.3)
So what? The plastic wrapper prevents the cell from shorting out ... right? Right. But only as long as the wrapper isn't damaged. Incovieniently, the tiny metal piece also does this (Pict. 4), damaging and stripping off the plastic foil when the cell is moving and turning in the holder. A real two-in-one disaster deal.
When the plastic foil eventually fails, the tiny metal piece makes contact with the negative rim, while also being connected to the positive terminal of the battery via the the positive contact of the holder, causing a short directly across the cell which no protection board - usually not even ones integrated into the end of the battery - can stop. This can of course be very dangerous.
Picture 4
An 18650 which had its plastic wrapper damaged by the metal piece.
The cell in Picture 4 experienced such an issue. It got very warm, luckily not too warm to touch. I placed it on the railing of the balcony while I went back inside where I took apart my headset, trying to find a short that could have caused it, and questioning the TP4056+Protection board, aswell as my soldering.
I am currently trying to find out where I used the 10 of these holders I bought. So far, I have found 8. I will of course be replacing them.